1. Field of the Invention
The present invention relates to a technique for preventing black level fluctuation due to clamp error in input picture signals, in a display device such as a liquid crystal projector or a computer display.
2. Description of the Related Art
Many display devices such as computer displays and liquid crystal projectors and the like have interface terminals for inputting analog picture signals (analog RGB signals and analog YCrCb signals). The input analog picture signals are converted into digital values at an internal AD conversion circuit, and subjected to signal processing.
FIGS. 8A and 8B illustrate signal processing performed up to the AD conversion circuit in a general display device. While such a display device would normally be configured from three equivalent circuits for each of R, G, and B, only one circuit is shown here.
As shown in FIG. 8A, the input picture signal Vi is subjected to DC component reproduction processing at a clamp circuit 102, which is upstream from an AD conversion processing circuit 101.
The left side of FIG. 8B shows the waveform before clamping of the input picture signal, and the right side thereof shows the waveform following clamping. The clamp circuit 102 takes the blanking period (BLK) of the input picture signal to forcibly change the signal level of the input picture signal to a clamp level (a level equivalent to level 0 following AC conversion, for example). Accordingly, as long as the blanking region of the input picture signal (equivalent to black signal) can always be clamped before AD conversion, the black signal is always digitized to black level (e.g., level 0).
However, in actual display devices, there is a phenomenon wherein the black level is not always reproduced as black, due to the following reasons, resulting in so-called black level fluctuation.
(1) Difference in DC component of input picture signals from the output device
(2) Clamp error at clamp circuit
(3) Offset error from sampling clock frequency in AD conversion
Feedback clamping has been proposed as a technique for solving the problem of black level fluctuation. Feedback clamping methods include a technique for comparing the black level in the analog signal format (See Japanese Patent Laid-Open No. 5-292538) and a technique for feedback to the clamp filter using data digitized by AD conversion (Japanese Patent Laid-Open No. 181939).
However, the former related art disclosed in Japanese Patent Laid-Open No. 5-292538 requires complicated analog signal processing circuits before AD conversion. This is in itself problematic, since a high-resolution display device requires 150 MHz or higher bandwidth for input analog picture signals. Accordingly, signal splitters and switching circuits and the like in the picture signal lines results in impedance matching problems, which in turn deteriorates the bandwidth of the picture signals, so the circuit before signal conversion should be as simple as possible.
On the other hand, the later related art Japanese Patent Laid-Open No. 9-181939 requires latch circuits or frame memory for always holding the digital level of the blanking portion, in addition to holding picture signals.
This method increases the frame memory installed, which is undesirable from a cost perspective as well. For example, in order to hold high-resolution picture signals of SXGA (1280×1024), the capacity of the frame memory required is 1280 (horizontal)×1024 (vertical)×8 (bit)×3 (colors), which is approximately 4 megabytes.
Also, further memory increases will be necessary to handle even higher resolution picture signals. Either that, or the signals must be compressed to below a certain capacity level, which could lead to deterioration of image quality. Further, complicated compression processing is required, which is undesirable from a circuit perspective. Accordingly, for all practical purposes, ensuring memory capacity for always storing the blanking region in addition to the picture signals is not realistic.